1. Field of the Invention
The present invention relates to a projection copying apparatus of a type in which an original image is projected on an exposure station through an optical system.
2. Related Background Art
Such an apparatus is used in a microfilm reader-printer and the like in which an image of an original such as a film is projected onto an exposure station through an image-forming lens system. Various image projection systems have been proposed for this type of apparatus.
In one of the known image projection systems (U.S. Pat. No. 4,367,033), three mirrors are arranged on the image field side of the image-forming lens to successively reflect a portion of the image-forming light beam so that an image of a slit-like portion of the original can be formed on a photosensitive medium in the exposure section. To perform the slit exposure, the first and second mirrors lying nearer to the lens are moved in a determined direction while moving the photosensitive medium passing through the exposure section at a constant speed.
This image projection system, however, has some problems. The first mirror which directly faces the image-forming lens is moved. Therefore, when the moving mirror is to be positioned close to the lens, the mirror is required to have a high degree of surface precision. If the precision of mirror surface is low, the image projected on the image plane is low in quality. Furthermore, as the first mirror, a large mirror is needed, which leads to a large size of apparatus as a whole. In addition, the oscillation of the first mirror has an adverse effect on the quality of images. Because of it, good quality cannot be attained for copied images with this known system.
It is also known to arrange not three but two mirrors on the image field side of the image-forming lens (German patent application Laid Open No. 2,915,512; Japanese patent application Laid Open No. 24,135/1983). However, like the first-mentioned prior art system, this prior art system also has the moving mirror at a position near the image-forming lens. Therefore, this known system has the same problems as described above.
In the conventional microfilm reader-printer, the optical path for magnifying and projecting an image of a microfilm onto a screen is called "reader optical path" and the optical path for projecting and exposing the image on a photosensitive medium to copy the image is called "printer optical path".
In some known microfilm reader-printers, a part of the reader optical path is used also as a part of the printer optical path (U.S. Pat. No. 4,367,033; German patent application Laid Open No. 2,915,512; U.S. Ser. No. 777,028).
In order to expose the image on the photosensitive medium, a scan-exposure method is often used according to which a scanning optical system constituting the printer optical path is moved to carry out scanning for exposure. When this scan-exposure method is employed in the above-mentioned type of reader-printer, there is produced a difficult problem. As the scanning optical system belonging to the printer section is moved, the printer section has to have a particular space used only for it independently of the space only for the, reader section. Because of these spaces, the apparatus, by necessity, has a large size. In the apparatus, the optical system belonging to the reader section and the scanning optical system belonging to the printer section are spaced a large distance from each other, which renders it very difficult to align the scanning direction of the scanning optical system with the optical system of the reader section. In the alignment, it is essential to hold a determined angular relation between the scanning direction and the reader optical system. If the moving direction of the scanning optical system is deviated from the direction of the determined angle and any deviation of the optical axis of the printer optical path is caused thereby, it is no longer possible to faithfully copy the image on a copy paper.
A known method for changing over the mode of the above-mentioned apparatus between reader mode and printer mode is to provide an optical path changeover mirror in the common optical path. The changeover mirror is turned out from and turned into the optical path for changing over the optical path between reader optical path and printer optical path. Obviously, this method needs a particular driving mechanism for rotationally moving the changeover mirror. It leads to a large, complicated and expensive apparatus.
A slit exposure type microfilm reader-printer is also known in the art in which two scanning mirrors are arranged in the printer optical path. A portion of the image-forming light beam is successively reflected by these scanning mirrors to form an image of a slit-like portion of the original on a photosensitive medium. The photosensitive medium is moved passing through the exposure section at a constant speed while moving the scanning mirrors in a determined direction to perform the slit exposure on the photosensitive medium. This type of reader-printer also has some drawbacks.
If the scanning mirrors run away accidentally during the reader mode time or if there is an timing lag between the moving of the scanning mirrors and the turning of the changeover mirror, any or all of the mirrors may be broken by collision between them or, in the worst case, the apparatus may be totally broken.